Methods for monitoring the insulation of DC and AC networks by applying a DC voltage relative to ground over a measurement period, tracking its behaviour over time as a leakage capacitance decays, and measuring an insulation resistance relative to ground from its behaviour, are well known.
However, as a result of the network leakage capacitances present and also various disturbances in the network to be monitored, these DC-based methods of measurement are often inaccurate or cannot be used. Consequently, in the accepted manner of known art, the recommendation is to execute a plurality of measurement intervals one after another and to average arithmetically the measured values thereby achieved, or to correlate them in another manner in order to derive better information concerning the insulation resistance relative to ground. In this regard reference is made to EP 0 654 673 B1, EP 1 586 910 B1 and DE 101 06 200 C1.
What is disadvantageous in the above-cited methods, however, is the fact that a plurality of measurement periods must be considered in order to derive a valid measured value. In one variant, a stable measurement pulse must be generated with extrapolation from intermediate values to the final value, and a comparison of the extrapolated final value with the measured final values must also be undertaken, this procedure being complicated and susceptible to disturbances. As soon as the comparison fails to provide an equality or a permissible deviation, the measurements must be repeated until valid measured values are present. In particular, in cases of highly fluctuating network loads or low frequency elements of interference in the network, high inductive or capacitive loads such as networks in which voltage regulation takes place, either the measurement time is lengthened as a function of the frequency and amplitude of the interference, or no valid measured values can be determined over acceptable measurement times.
In recent times the number of motor applications controlled by a converter, in which single-phase or three-phase AC synchronous or asynchronous machines find application, has been increasing strongly. In these networks, as a result of the low switching frequency and also the calibration and control processes of the converter, low frequencies are to be anticipated for the interference and fluctuations in voltage and current values, such that conventional DC-based methods of measurement to determine the insulation measured value are almost impossible to deploy in practice. Therefore such methods can only be deployed to determine insulation faults under operating conditions with a stable voltage, for example, in the stationary state or in defined motor states, but under no circumstances in cases of dynamic load regulation.
On the other hand, methods for measuring the impedance between phase and earth are also of known prior art. In these methods, a sinusoidal measurement voltage is applied between an insulated system and ground, so that the frequency of the measurement signal is in a fixed ratio to the frequency of the network voltage.
Thus, for example, a microcontroller-implemented frequency-selective measurement method emerges from U.S. Pat. No. 5,450,328 in which a network leakage impedance Z is calculated from a measurement frequency and phase matched to the network frequency; from which ohmic and capacitive components can be derived. In this context, two measurement voltage sources generate two measurement voltages displaced by 180° into the two phases of an AC voltage network, which are coupled into the network, so that a frequency-selective voltmeter decouples measurement voltages and currents from the network. However, this method has the disadvantage that it renders necessary an increased level of measurement effort as well as complex signal recording, so that, in the case of high leakage capacities, only a very inaccurate estimation of the leakage resistance can be achieved.
What is disadvantageous in all of the above-cited methods is the fact that, particularly in networks with high dynamic loads such as converter circuits for the operation of motors deployed in hybrid electric vehicles, only inaccurate measurement results can be determined. Essentially, no measurements for monitoring the insulation resistance relative to ground can be undertaken.